1. Field of the Invention
The present invention relates to an over-voltage suppression apparatus that suppresses over-voltage generated when a circuit breaker is re-closed.
2. Description of the Related Art
In general, on a no-load transmission line in which no compensation by a reactor is applied, there is a residual DC voltage on the transmission line after the circuit breaker interrupts the current. As is known, if the circuit breaker is re-closed in a condition in which this DC voltage is still present, an over-voltage (connection surge) is generated. The magnitude of this over-voltage is several times the system voltage. There is a risk that generation of such a large over-voltage may affect the insulation of equipment installed in the system.
A known method of suppressing such over-voltage when re-closing of a no-load transmission line is effected is the provision of a circuit breaker fitted with a resistor. For example in the case of a 500 kV system as used in Japan, a circuit breaker of the type that introduces a resistance into the circuit is employed in order to suppress such over-voltage. A circuit breaker fitted with a resistor has a construction in which the resistor that is introduced is connected in series with the contact. In a circuit breaker fitted with a resistor, connection is effected in parallel with the main contacts of the circuit breaker. A circuit breaker fitted with a resistor is re-closed before reclosing the main contacts of the circuit breaker. In this way, over-voltage is suppressed. An example is described in “Practical and Theoretical Handbook of Power System Technology” by Yoshihide Hase (hereinafter referred to as Non-patent Reference 1).
In contrast, in the case of a no-load transmission line that is compensated by a reactor, after current interruption is effected by the circuit breaker, an oscillating voltage is generated on the transmission line by the electrostatic capacitance thereof and the reactor. Even in this case, over-voltage is generated if the circuit breaker is re-closed at a time-point where the voltage between the circuit breaker contacts is large. In order to suppress over-voltage when re-closing a transmission line that is compensated by a reactor, a known method is to control the phase (timing) at which the circuit breaker is closed. This method consists in performing re-closing of the circuit breaker at a time-point where the voltage between contacts is small. The following are known methods of predicting the time-point at which the voltage between contacts is small.
As a first method, a method in which the voltage between contacts of the circuit breaker is approximated by a function, and the circuit breaker is closed with optimum timing is disclosed as follows. Let us first assume that the power source (side) voltage is a sine-wave of mains frequency. Also, if the oscillation voltage on the line side is of a single frequency, it can be regarded as a sine-wave. The voltage between contacts is predicted by approximating these two voltages by a sine-wave function. The closure timing of the circuit breaker is determined using this voltage between contacts. An example is to be found in Laid-open Japanese Patent Publication Tokkai 2003-168335 (hereinafter referred to as Patent Reference 1).
As the second method, a method in which the time between zero-points of voltage between contacts of the circuit breaker is measured and, using this information, the circuit breaker is closed at a future zero-point voltage between contacts of the circuit breaker is disclosed as follows. In this method, the time between the voltage zero points of a single cycle of the voltage between contacts after circuit breaking and the time between voltage zero points of the next single cycle of the voltage between contacts are measured. If these two times between the zero points of the voltage between contacts are the same, the frequency of the voltage between contacts is known. In this way, the future zero-point of the voltage between contacts can be deduced irrespective of the voltage waveform. An example is to be found in K. Froehlich: “Controlled Closing on Shunt Reactor Compensated Transmission Lines Part I: Closing Control Device Development”, IEEE Transactions on Power Delivery, The Institute of Electrical and Electronics Engineers, Inc., April 1997, Vol. 12, No. 2, p 734-740 (hereinafter referred to as Non-patent Reference 2).
However, there are the following respective problems with the methods of over-voltage suppression described above.
If the method of over-voltage suppression using a circuit breaker fitted with a resistor is employed, a circuit breaker fitted with a resistor must be specially added to an ordinary circuit breaker. Consequently, in terms of the circuit breaker as a whole, the circuit breaker size is increased.
In some cases, a reactor is installed on the transmission line in order to compensate reactive power. When the transmission line on which the reactor is installed is open-circuited by the circuit breaker, voltage oscillations of the frequency determined by the electrostatic capacity of the transmission line and the inductance of the reactor are generated on the transmission line. In general, the frequency of the voltage oscillations of the transmission line is different from the frequency of the power source voltage. In this case, the voltage between contacts of the circuit breaker has a multifrequency wave (or multiple frequency wave).
In determining the optimum closure timing for a circuit breaker by approximating the voltage between contacts of the circuit breaker by a function, there are the following problems.
The electrostatic capacity of a transmission line, which determines the frequency of voltage oscillations of the line, comprises an in-phase capacitative component with respect to ground, an inter-phase component between the phase in question and other phases, and a component of the other phases with respect to ground. These electrostatic capacitances have different values in each phase, depending on the geometrical arrangement of the transmission line. Consequently, it is extremely rare for the oscillation waveform of the line voltage to be a single-frequency sine wave. Frequently, this oscillation waveform is itself already a multifrequency waveform. In this case, it is in itself difficult to approximate the voltage oscillations of the line by a function. Accordingly, it is extremely difficult in practice to find the voltage between contacts from a function approximation.
Furthermore, the following problems are experienced if the timing for circuit breaker closure is obtained by measuring the time between the voltage between contacts between zero points of the circuit breaker.
If the circuit breaker is closed in a condition with voltage applied between the circuit breaker poles, a discharge will be generated between the contacts if the voltage between the contacts exceeds the voltage-withstanding capability (dielectric strength) of the insulation between the contacts. If such a discharge is generated, the circuit breaker is brought into an electrically contacting condition before mechanical contact of the contacts takes place. Such a discharge is termed “pre-arcing”.
Now if the voltage between contacts of the circuit breaker is a multifrequency waveform, this voltage may have a peak value (crest value) greater than the power source voltage. In such cases, it can happen that a closed condition is produced by discharge produced by pre-arcing as described above at a time-point where the voltage between contacts is large, even though the circuit breaker attempted to close at a zero-point of the voltage between contacts.
In such cases, a large over-voltage can be generated. Consequently, when the voltage between contacts is of multifrequency waveform, over-voltage cannot be suppressed purely by measuring the voltage between contacts zero-points.